Measurement of Plutonium Contamination Through Paint Using a Fidler Probe

Alpha contamination detection usually relies on good surface conditions — unobstructed, smooth, clean and flat. However, in many circumstances, the conditions may be difficult, or it may be that there is a significant chance that contamination has been deliberately or accidentally painted over. Hence an alternative method of monitoring would be useful. In some areas of fuel manufacture and reprocessing, and in weapons manufacture, potential contamination is dominated by plutonium isotopes and by ingrown Am-241, derived from the beta decay of Pu-241. Plutonium is often thought of as a mainly an alpha emitter, together with a few weak betas detectable by direct means only with extreme difficulty. However, the alpha emitting isotopes of plutonium also emit significant L x-rays in the 11 to 20 keV energy range, as does Am-241, which also emits a 60 keV gamma with a 36% probability. These X-rays are unattenuated to any extent in air over a range of 1 metre. They also penetrate paint significantly, which makes them detectable by a suitable probe. The Fidler probe was designed as an efficient detector of these X and gamma radiations. The window is thin beryllium and the scintillator is thin sodium iodide. This leads to a very efficient detection of both the X-rays in question and the 60 keV gamma radiation while keeping the background as low as possible. The signal from such a detector can be processed in several ways — gross counting above a threshold, counting in regions of interest or full spectrometry. The advantages of the latter include the minimisation of background, easy background correction, the ability to use the recorded X-ray spectrum to correct the measured counts and the identification of the presence of other gamma emitters. Nuvia Ltd have been developing techniques to look at the spectrum from Fidler probes and derive a calibration factor, either from using the 60 keV gamma measurement only where the fingerprint is well known and stable, or by using samples of paint and small area sources to actually measure the transmission factor where the Am-241 fraction is variable. Using these techniques, it is possible to obtain a good estimate of the plutonium contamination level under paint over a range of conditions.